1. Field of the Invention
The present invention relates to a method for making a solid electrolytic capacitor by utilizing a porous sintered body made of a valve metal.
2. Description of the Related Art
Conventionally, to remove noises generated from a device such as a CPU or stabilize the power supply system of an electronic device, solid electrolytic capacitors are widely used, each including a porous sintered body made of a metal material having valve action (hereinafter simply referred to as “valve metal”).
FIGS. 23–26 show an example of conventionally proposed method for making such a solid electrolytic capacitor (see JP-A 2003-77769). As shown in FIG. 23, this manufacturing method uses an apparatus provided with a mold B2 which includes a stationary block 111, four pressing blocks 112A–112D, and a movable block 113. The stationary block 111 and the four pressing blocks 112A–112D define a cavity 117, and a wire 105 is arranged in the cavity. Subsequently, as shown in FIG. 24, valve metal powder 104 is loaded into the cavity 117, and then the powder 104 is covered by the movable block 113. Subsequently, as shown in FIGS. 25 and 26, the four pressing blocks 112A–112D are moved toward the center of the cavity 117 to compact the powder 104 from four sides, thereby providing a porous body. The porous body is then heated and sintered to provide a porous sintered body. By using the porous sintered boy, a solid electrolytic capacitor is made.
Recently, in accordance with an increase in the clock speed of CPUs and in operation speed of electronic devices as well as digitalization of electronic devices, various requirements are placed on capacitors. For example, noise cancellation property for a wide frequency band is demanded. Further, high responsiveness with respect to high frequencies and capability of high power supply are demanded. For fulfilling these demands, to increase the capacitance and to reduce the resistance and impedance of a capacitor are effective.
As a method for increasing the capacitance of a capacitor, it may be considered to make a capacitor by using a plurality of porous bodies. With this method, however, the manufacturing process becomes complicated, and the size of a capacitor increases. It may be considered to attain a desired capacitance by connecting a plurality of capacitors in parallel. In such a case, however, a large number of wires need be used for passing current to the capacitors, and the current path becomes long. As a result, the method poses problems such as an increase in impedance in the wiring pattern, a deterioration in the space efficiency on the substrate and an increase in the manufacturing cost caused by an increase in the number of parts.
In light of the above, it is desirable that the capacitance is increased while using a single capacitor and without increasing the size of the capacitor. In the case of a capacitor made by using a porous sintered body, the capacitance can be increased by increasing the size and density of the porous sintered body and without increasing the size of the capacitor itself.
However, when the size of a porous sintered body increases, the equivalent series resistance and equivalent series inductance increase. To avoid such disadvantages, it may be considered to make the porous sintered body flat. When the porous sintered body is flat, the path of the current flowing through the porous body becomes short. Since the surface area of the porous sintered body increases by making the body flat, the contact area with a graphite layer and a silver layer provided on a surface of the porous sintered body increases. As a result, the resistance and impedance are reduced. Moreover, a capacitor made by using the flat porous sintered body has a relatively small height, which is advantageous for reducing the thickness of the device in which the capacitor is incorporated.
In this way, to fulfill the demands such as an enhanced noise cancellation property for a high frequency band, capability of high power supply and high responsiveness with respect to high frequencies, it is desirable to increase the size and density of the porous sintered body while making the porous sintered body flat.
However, the prior art manufacturing method shown in FIG. 23 has the following problems in forming a large, high-density and flat porous body.
First, to make a large porous body, the volume of the cavity 117 need made large. Accordingly, the travel distance of the four pressing blocks 112A–112D becomes long. Further, to form a porous body having a high density, the ratio of the volumes of the cavity 117 before and after the compacting need be large. For this reason again, the travel distance of the pressing blocks 112A–112D becomes long. As the travel distance of the four pressing blocks 112A–112D increases, the center portion and the peripheral portion of the porous body may differ in distribution density of the powder 104. When the distribution density of the powder 104 is non-uniform in a flat porous body, excessive deformation or breakage due to the insufficient strength may occur in heating and sintering the porous body. Further, when the heating and sintering of the porous body cannot be properly performed due to the non-uniformity in the porous body, a leakage current in the capacitor manufactured by using the porous body increases. Moreover, when the density of the powder 104 is non-uniform, the bonding between the wire 105 arranged in the porous body and the powder 104 becomes insufficient, which may increase the contact resistance between these portions.
Second, in the prior art manufacturing method, the four pressing blocks 112A–112D are used to compact the powder 104 in a direction crossing the thickness direction of the porous body. Therefore, among the obverse surfaces of the porous body, the upper and the lower surfaces oriented in the thickness direction are rubbed against the relevant surfaces of the stationary block 111 and the movable block 113. When the travel distance of the four pressing blocks 112A–112D are increased for increasing the size and density of the porous body, the distance through which the upper and the lower surfaces are rubbed increases. Further, to make a porous body having a high density, the powder need be compacted with a high compressive force, so that the contact pressure between the upper and the lower surfaces and the stationary block 111 and the movable block 113 increases. Therefore, the minute pores existing in the upper and the lower surfaces are liable to be closed or clogged due to the friction. Thus, in the prior art method, it is difficult to form a porous body which is larger than a certain size without causing clogging.
When clogging occurs at a surface of the porous body, an aqueous solution of phosphoric acid for forming a dielectric layer and an aqueous solution of manganese dioxide for forming an electrolyte layer, for example, may not be duly impregnated through the surface and into the porous sintered body made by heating and sintering the porous body. As the porous sintered body becomes flatter, the proportion of the area of the upper and the lower surfaces to the total surface area of the porous sintered body increases. This makes higher the possibility that the formation of the dielectric layer and the electrolyte layer in the porous sintered body is hindered. In such a case, the capacitor cannot duly function as a solid electrolytic capacitor having a polarity. Moreover, when clogging occurs, the contact area between e.g. the graphite layer and the silver layer formed on the surface of the porous sintered body and the electrolyte layer formed at the porous sintered body reduces, whereby the contact resistance between these may increase.